A magneto-optical biosensor for detecting magnetic beads at several binding spots on an optical interface using frustrated total internal reflection (FTIR) can be used as a sensing system for sensing a substance in a fluid. For performing FTIR light from a light source is directed onto the optical interface such that an evanescent field is generated at the binding spots. The magnetic beads at the binding spots influence the evanescent field, wherein this influence is detected as intensity variations in the light reflected from the optical interface. The detected intensity variations are used for determining the concentration of the magnetic beads at the binding spots. By binding or non-binding of these magnetic beads to the optical interface in a biological assay, the presence of various substances, e.g. drugs-of-abuse or cardiac troponin-I, are detected in real matrices like saliva or blood by detecting the magnetic beads, which have attached the substances, within the evanescent field. One dedicated area on the biosensor optical interface is modified such that magnetic beads cannot reach the evanescent field and the total internal reflection is not frustrated, thereby keeping the detected signal at its maximum, i.e. reference, level. This so-called True-White-Reference (TWR) is used to suppress by normalization common-mode intensity-variations in each of the binding spots induced by, for example, the light source.